They worked Mike as they were actually dyno-tested back when they were popular. They actually snap in as to raise the port floors. On the exhaust side, they were more of use raising the floor due to FORD exhaust runners being squeezed by need of them fitting into the uniboby chassis and fitting exhaust manifolds to them (same on 385 Series).

I had a 73 RANCHERO with the 351CJ and that thing ran like a scalded ape (it was bone stock).